Did flooding drown soybean cyst nematode?

Figure 1. The aftermath of flooding, Union County. Flood water carried soil and debris from one point to another and may in the process also spread SCN.

Photo by E. Byamukama

This has been a wet June with some areas receiving record shattering rainfall. The inclement weather led to many soybean fields being flooded or ponded out (Figure 1). What effect will the flooding have on soybean cyst nematode (SCN) in the field and even fields surrounding an infested field?

Eggs and juveniles inside the SCN cyst are protected from harsh weather conditions (drought and floods) and extreme temperatures (from -11° F to 104° F). However, their ability to emerge and cause new infections is greatly limited under these less than ideal conditions. Eggs and juveniles outside the cyst do not survive for very long.

At the time of heavy rainfall, soybean plants were between growth stages V1 and V3. For soybean fields that had SCN present, infection may have already taken place (Figure 2). Flooding may not have impacted the level of infection but because of these wet conditions, we may not see severe symptoms. SCN thrives under warm and dry conditions after infecting the soybean root.

Flooding most likely has moved SCN around. Flood water carries top soil with it and if the soil is infested, the SCN may be moved to previously uninfested areas. Anything that moves soil (e.g. cultivation equipment, boots, wind erosion, animals) also moves SCN.

Once a field is infested with SCN, it is impossible to eradicate and it can only be managed to help its numbers below damaging levels. Because soybeans can be infected without showing symptoms, testing soil for SCN is more reliable than looking for symptoms in the field. Growers should sample their soybean fields and have the soil sample tested for SCN to ascertain their presence. For fields that have already been found with SCN, continued testing is needed to determine any changes in the numbers. Increased SCN populations in a field may indicate that the management techniques being used, such as resistant cultivars, are not working well. Changes may be needed, such as longer rotations out of soybeans. If SCN is confirmed, growers should combine several tactics to prevent the population from increasing including planting resistant cultivars, crop rotation with non-SCN hosts, rotating within resistant cultivars, and nematicidal seed treatments (See: Soybean Cyst Nematode: One cyst is the threshold for action).